This invention relates to a process for the production of an aluminum grain refiner and, more specifically, to an Al-Ti-B grain refiner.
Typically, aluminum grain refiner alloys of the type contemplated by the present invention consist essentially of 2-12 wt % titanium, either alone or together with 0.1-2 wt % boron, and the balance being commercial grade aluminum with normal impurities. Such Al-Ti-B grain refiner alloys are conventionally produced batchwise in an electric induction furnace. The alloying ingredients are typically provided in the form of metal salts preferably in the form of the double fluoride salts of titanium and boron with potassium.
In the typical batch process, a mixture of fluoride salts in the required proportion is fed to a stirred body of molten aluminum in an induction furnace at a temperature in the range of about 700.degree.-800.degree. C. By means of an electro-magnetic stirring action, the salt mixture is drawn below the surface of the melt where a reduction to Ti and B by the Al takes placed. This alloying reaction results in a product which comprises molten potassium alumium fluoride. Periodically during the alloying process, and at the end of the process, electric power is shut off to allow the molten reaction products to rise to the surface of the molten metal where they form a discrete slag layer. This slag layer is removed by decanting into a suitable receptable, such as a slag pan.
The batch of molten alloy thus obtained may be transferred to a separate casting furnace. This is typically an electric induction furnace in which electro-magnetic stirring helps to keep the insoluble TiB.sub.2 particles suspended within the molten alloy body. The alloy may be cast into either an ingot for further working to rod by rolling or by extruding or directly into a rod casting machine, such as a Properzi caster.
The above known process has a number of significant disadvantages. Firstly, the product quality, particularly microstructure and grain refining properties, varies from batch to batch. Secondly, the alloying process produces environmentally damaging fluoride-containing fumes in the form of intense emissions for a short period of time and this necessitates an expensive emission control system large enough to handle the periodic high emission rates. Thirdly, the system is very capital intensive.
It is known to use continuous alloying processes utilizing a flowing stream of molten metal. For instance, U.S. Pat. No. 4,298,377 discloses a method and apparatus for adding solids to molten metal by continuously feeding both the solids and the metal into a vortex-forming chamber from which the mixture is discharged at the core of the vortex as a free-falling, hollow-centered stream.
U.S. Pat. No. 3,272,617 discloses a method and apparatus for continuosly pouring a stream of molten metal to form a vortex into which a particulate alloying agent is introduced and where the intensity of the vortex is controlled to immerse the additives in the molten metal at any desired rate.
Another method and apparatus are disclosed in U.S. Pat. No. 4,484,731 for continuously treating molten metal with a treatment agent which is continuously introduced into a treating vessel through a supply passage formed through the wall of the vessel. The molten metal is continuously poured into the lip of the vessel and discharged from the lower part of the vessel after addition of the treating agent.
The above techniques involve total mixing of the reactants into a stirred body of molten metal. This creates a significant problem in that the final grain refiner alloy may be contaminated by entrapped globules of molten salt reaction product. It is, therefore, the object of the present invention to provide an improved process for contacting molten aluminum with grain refining compounds while avoiding the above problem of entrapped globules.